TBM excavation was recently completed for the twin drives below Mont Sion, in south west France, for the A41 highway extension. A single shield of 11.94m diameter was used to bore both tubes for the fast-track construction programme being executed by a Bouygues-led JV, called GIE Constructers A41.

The JV excavated the tunnels through molasse deposits with little groundwater under Mont Sion. Both tubes are approximately 3km long and 10.7m i.d. built of concrete segments (5+1) in 2m long rings.

Excavation began in October 2006 and the first tube had been completed by June 2007 – after which the JV took the cutterhead in its entirety back for reassembly and relaunch last September.

Following the completion of the main excavation with the Herrenknecht TBM (S-333), in March, work is currently underway on eight connecting passages (four vehicular, four pedestrian). There will also be 20 emergency and 15 fire equipment recesses.

The project, being built for a 55-year concession, is designed by a JV of Bouygues and SETEC. The entire construction schedule is 29 months, concurrent engineering has been key to pushing ahead the pace, and the road in the French Alps is due to be opened just before the end of the year.

GIE Constructers celebrate final breakthrough